[REVIEW] Change python docs to pydata theme

[galipremsagar]

This PR changes docs theme from sphinx_rtd_theme to use pydata-theme: https://pydata-sphinx-theme.readthedocs.io/en/latest/

[codecov-commenter]

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@@               Coverage Diff               @@
##             branch-21.10    #1785   +/-   ##
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  Coverage                ?   59.68%           
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  Files                   ?       77           
  Lines                   ?     3567           
  Branches                ?        0           
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  Hits                    ?     2129           
  Misses                  ?     1438           
  Partials                ?        0           

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[BradReesWork]

@gpucibot merge